1. Field of the Invention
This invention relates to the continuous manufacture of films and sheets of polyolefins, especially polypropylene, by means of calendering, i.e. by squeezing a mass of heat-softened thermoplastic polyolefin between two or more heated, horizontal, parallel rollers.
The process of calendering was originally developed in the rubber industry for the continuous manufacture of natural and synthetic rubber sheeting, and was later adapted to the manufacture of films and sheets of synthetic thermoplastics such as vinyl chloride homopolymers and copolymers, cellulose acetate, and, to a limited extent, polyethylene and other polyolefins. In the case of polyolefins, and of polypropylene in particular, the overwhelming preponderance of film and sheeting is manufactured by processes other than calendering, such as casting, blow extrusion (for film, having a thickness of 10 mils and under), and flat-die extrusion (for sheet, having a thickness of over 10 mils). As used herein the term polypropylene includes both homopolymers and random and block copolymers of propylene and another olefin such as ethylene or butene-1.
Since its commercial introduction in 1958 polypropylene, especially highly isotactic polypropylene, has had a rapid growth rate and is presently produced at an annual rate of almost four billion pounds, due to a combination of raw material availability, low cost, low specific gravity, and many desirable chemical and physical properties. Among the latter can be mentioned: resistance to many solvents, clarity and gloss, ability to be colored, ability to be electroplated and vacuum metallized, high tensile yield strength, stiffness, relatively high heat distortion, temperature, heat resistance, and excellent resistance to environmental stress cracking. Of the approximately four billion pounds made annually, more than three hundred million pounds per year are converted to film and sheet, with the balance being mainly injection molded or extruded into fibers, pipe, and other profiles. The film and sheet is used for a variety of purposes, which include: bread wrap and other food packaging applications, including boilable food pouches; appliance housings; various automotive applications such as kick panels, instrument panels, and trim; and numerous others. Although various disclosures have been made of the manufacture of polypropylene film and sheeting by means of calendering, this process has not achieved any substantial commercial success despite the fact that it does have potential advantages over blow extrusion and flat-die extrusion. One such advantage is the fact that during production runs color changes can be made easily without extensive clean-out of the equipment. Another advantage is that calendering can provide better control of thickness variation across the width of the sheet as compared with flat-die extrusion.
A major problem which has held back the calendering of polypropylene on a large scale is the tendency of this polymer, which has a relatively sharp melting point, to stick tenaciously to the calender roll surfaces. This makes it difficult or impossible to strip the film or sheet from the last roll at a temperature high enough to retain the desired surface smoothness. Although some limited success, has been achieved in producing a smooth sheet of polypropylene on a calender, it was done at such a low production rate due to the lack of a suitable lubricant for the polymer as to make the process economically unacceptable. Increasing the production rate resulted in a sheet totally lacking in surface smoothness. In varying degrees, the same problems occur with other polyolefins.
Because of the combination of chemical and physical properties possessed by propylene homopolymers and copolymers as previously mentioned, these materials are potentially uniquely suitable for the manufacture of containers (trays, blisters, tubs, cups, and lids, by way of examples), especially containers for packaging food and drug products, by means of thermoforming, and especially vacuum forming. The lack of polypropylene sheets having uniform thickness, smooth surfaces, and good clarity has thus far hampered the fulfillment of this potential.
It is an objective of this invention to overcome the problems encountered in calendering polypropylene and other polyolefins through the use of a novel combination of lubricants which is incorporated into the polymer to be calendered.
2. Description of the Prior Art
U.S. Pat. No. 2,462,331 discloses a calenderable composition comprising polyethylene and, as a lubricant, a polyhydric alcohol ester of, or a metal salt of, a monocarboxylic fatty acid having from 12 to 24 carbon atoms.
U.S. Pat. No. 3,007,207 discloses a process for calendering crystalline organic polymers, including polyethylene and polypropylene. The polymer composition being calendered can contain an antioxidant.
U.S. Pat. No. 3,176,021 discloses a process for melt extruding a composition comprising polypropylene and an amide of a water-insoluble monocarboxylic acid having from 8 to 18 carbon atoms.
U.S. Pat. No. 3,184,526 discloses a process for producing self-supporting films of solid polyolefins, including polyethylene and polypropylene, using a pair of closely spaced rotating rolls. The composition used to form the films comprises fine particles of polyolefin wet with an inert organic diluent.
U.S. Pat. No. 3,248,351 discloses a process for improving the color of polyolefins, including polyethylene and polypropylene, by mixing polyolefin wet with methanol with the zinc salt of a monocarboxylic acid having 1 to 12 carbon atoms and with an oxidation stabilizing compound.
U.S. Pat. No. 3,320,334 discloses polyolefin compositions rendered dyereceptive by having incorporated into the polyolefin a pyridinic polyamide poly-condensation product of isocinchomeronic acid or its esters with an aliphatic diamine. Suitable polyolefins include polyethylene and polypropylene.
U.S. Pat. No. 3,328,503 discloses a process for forming oriented crystalline thermoplastic film and sheeting which comprises passing a preformed web of molten amorphous polymer through the nip of a pair of counter-rotating rolls without forming a bank at the ingress side. The apparatus used can be a calender, and the polymer can be polyethylene or polypropylene. A combination of calcium stearate, stearic acid, and dibutylparacresol is disclosed as a calendering aid for polyethylene.
U.S. Pat. No. 4,144,304 discloses a process for calendering polyolefins, including polyethylene and polypropylene, wherein the polyolefin is admixed with vegetable fibers. The presence of vegetable fibers prevents the mixture from sticking to the calender rolls. The mixture can also contain a lubricant, such as calcium stearate.
U.S. Pat. No. 4,251,407 discloses the use of zinc oxide as an anti-corrosive additive in polypropylene compositions which contain a fatty acid salt, such as calcium stearate, as an acid acceptor.
A discussion of the problem of surface irregularities of calendered polypropylene was presented by Prentice, in Polymer, 1981, Vol. 22 p. 250-4.